Activation ofShigella flexneritype 3 secretion requires a host-induced conformational change to the translocon pore

AbstractType 3 secretion systems (T3SSs) are conserved bacterial nanomachines that inject virulence proteins (effectors) into eukaryotic cells during infection. Due to their ability to introduce heterologous protein into human cells, these systems are being developed as therapeutic delivery devices. The T3SS assembles a translocon pore in the plasma membrane and then docks onto the pore. Docking activates effector secretion through the pore and into the host cytosol. Here, usingShigella flexneri, a model pathogen for the study of type 3 secretion, we determined the molecular mechanisms by which host intermediate filaments trigger docking and enable effector secretion. We show that the interaction of intermediate filaments with the translocon pore protein IpaC changed the pore’s conformation in a manner that was required for docking. Intermediate filaments repositioned residues of theShigellapore protein IpaC that are located on the surface of the pore and in the pore channel. Restricting these conformational changes blocked docking in an intermediate filament-dependent manner. These data demonstrate that a host-induced conformational change to the pore enables T3SS docking and effector secretion, providing new mechanistic insight into the regulation of type 3 secretion.Author summaryThe movement of bacterial proteins across membranes is essential for bacterial physiology and bacterial virulence. The type 3 secretion system moves bacterial virulence proteins from the inside of bacterial pathogens into human cells. To do so, the type 3 secretion system forms a pore in the plasma membrane of the target cell, attaches (docks) onto the pore, and delivers virulence proteins through the pore. Docking is essential for establishing a continuous channel from the inside of the bacteria to the inside of the human cell. What enables the type 3 secretion system to dock onto pores is not understood. We show that structural proteins in human cells, intermediate filament proteins, induce structural rearrangements to the type 3 secretion pore that trigger docking and that enable the subsequent delivery of virulence proteins into human cells. Due to the wide prevalence of type 3 secretion systems among human pathogens, these findings are likely to broadly enhance our understanding of type 3 secretion.